Photoconductive element for use in electrophotography containing a heavy metal soap of a long chain fatty acid; and process for using same



Oct 1965 D. G. KIMBLE ETAL 3,212,890

PHOTOCONDUCTIVE ELEMENT FOR USE IN ELECTROPHOTOGRAPHY CONTAINING A HEAVYMETAL SOAP OF A LONG CHAIN FATTY ACID; AND PROCESS FOR USING SAMEOriginal Filed Sept. 50, 1955 4- INVENTORS DONAL (7'. KIMBLE C LflAE/VCE 0. MOP/457E? BYRON W IVEHER 5) WZW United States Patent 3,212,890PHOTOCONDUCTIVE ELEMENT FOR USE IN ELECTROPHOTOGRAPHY CONTAINIVG A HEAVYMETAL SOAP OF A LONG CHAIN FATTY ACID; AND PROCESS FOR USING SAME DonalG. Kimble, Maplewood, and Clarence O. McMaster, St. Paul, Minn., andByron W. Neher, Hudson, Wis., assignors to Minnesota Mining &Manufacturing Company, St. Paul, Minn., a corporation of DelawareOriginal application Sept. 30, 1955, Ser. No. 537,647, now Patent No.3,138,458, dated June 23, 1964. Divided and this application Feb. 26,1964, Ser. No. 347,526 9 Claims. (Cl, 96-1) This invention is primarilyconcerned with the printing or permanentizing of light-images, opticallyproduced in accordance with well-known principles employed inphotography or photo-copying, by a dynamic electrophotographic processemploying copying-paper or the like having a photoelectrosensitivesurface. The process involves establishing a high potential differencebetween the copypaper and a coma electrode while the sheet is under theinfluence of a light-image, and then passing an aerosol developing agentacross the face of the sheet while maintaining a high potential at thesheet or the electrode or both. The invention concerns the copy-sheetand the equipment and apparatus employed, as wellas the novel dynamiccopying-process. This application is a division of our prior andcopending application S.N. 537,647, filed September 30, 1955.

Accordingly, one of the objects of the invention is the provision ofnovel and useful methods and processes for the printing of light-images.Another object is the provision of apparatus suitable for carrying outthe novel processes. A further object is the provision of materials foruse in conjunction with said apparatus in producing visible copies oflight-images by said methods. A still further object is the provision ofcopy-sheet material adapted to the preparation of permanent,smudge-resistant copies of impressed light-images, including multicolorimages, by methods here described. A specific object is the dynamicdevelopment on such copy-sheets of both direct and reverse visiblecopies having extremely high contrast, by methods involving continuousor intermittent deposition of any desired amount of the developermaterial. Other specific objects are the provision of methods for theprinting of light-images under adverse humidity conditions; in acontinuous rapid manner, e.g., by simultaneous exposure and development;and with the production of continuous-tone images and of uniform densitysolid image areas. A still further object is the provision of methodspermitting the production of visible images on poorly insulativephoto-electrosensitive sheet materials and with a wide variety ofdeveloper materials including both readily charged and difiicultlycharged developer materials. Another object is to produce prints havinghigh resolution and freedom from smudging, and by simplified and rapidprocedures.

The present invention accomplishes these and other objects and providesnumerous advantages, as will be made apparent or will be specificallypointed out hereinafter.

In the accompanying drawing:

FIGURE 1 illustrates in perspective, and largely schematically, theminimum requirements of the apparatus for carrying out the invention;

FIGURE 2 illustrates in cross-section a preferred type of copying-paperemployed in the process of the invention;

FIGURE 3 illustates in cross-section a preferred source ofimageproducing components; and

3,212,890 Patented Oct. 19, 1965 FIGURES 4 and 5 schematicallyillustrate details of alternative structure employed in the apparatus ofFIG- URE 1.

The apparatus depicted in FIGURE 1 comprises an image source 10 fromwhich the image is projected, through a filter 15, if desired, past acorona source 11 and against a suitable receptor 12, in this instance aflexible copying-paper held in contact with a conductive support 13. Ahigh voltage direct current power supply is connected between the coronasource 11 and the support 13 or the receptor sheet 12. An element=14'providing a source of image-producing developer components islocated adjacent the lower front surface of the copy-sheet 12.

A light-image is projected on the receptor 12 from the projector 10, asuitable image-producing developer component is released from the source-14 in the form of an aerosol, i.e., a cloud of extremely fine particlessuspended in the air, and a suitable high potential difference isestablished between the plate 13 and the corona source element 11. Thecombination of the light-image on the receptor 12 and the high potentialdifference between the plate and the point source 11 causes particles tobe pre erentially and continuously transposed from the aerosol to imageareas of the receptor, where they are deposited and retained to form acopy of the initially applied lightimage. Where contrasting dye or colorparticles form the image-producing developer component, the copyproduced is directly visible.

It has been found that the process just described is capable ofproducing completed prints within surprisingly short periods of time.The prints are of exceptional clarity, showing the finest detail whichit has been possible to project by high quality optical systems.Permanent prints may be made directly, without any need for subsequentfixing or processing. Depending on the character of the componentsemployed, the print may be made in any desired color or combination ofcolors, including black and white. Printed areas, such as line or blockareas, are uniformly colored over their entire area rather than beingintensified along edge areas as in many electrical printing processes.Printing may be continued to any desired thickness of the developermaterial. Prints capable of serving as master copies or asprinting-plates, for subsequent reproduction by well known duplicatingmethods such as dye transfer or lithography, are easily made.

It is to be understood that each of the several components of theapparatus indicated in FIGURE 1 may be replaced by any of a variety ofequivalent components providing equivalent functions and operating onthe same principles. Further elaboration of these principles will now beprovided.

The projector 10 of FIGURE 1 includes a source of light 101, atransparency 102 carrying the graphic subject matter and through whichthe light is directed, and a lens system 163 for focusing the resultingmodified light beam on the surface of the receptor 12. Such devices arein common use as slide projectors and for photo enlarging. A sharp imageof high contrast is provided. The intensity and duration of exposure iseasily controlled. Analogous systems reflect the image from an opaqueprint. as in photocopying, or transmit an image from a lighted subject,as in photography. These and other optical systems may replace theprojector 10 of FIGURE 1, provided suflicient space is made availableadjacent the irradiated surface for establishing an effective corona andfor uniformly introducing and distributing the imageproducing aerosol.It will be apparent that excessive extraneous light must be eliminatedin order to provide the required image pattern and intensity at thesurface of the receptor.

The term light as applied to the radiation provided from the projectorwill be understood to include monochromatic light and various invisibleradiations as well as the white light ordinarily employed for imageprojection. Infrared radiations have been found useful in someinstances. Ultnaviolet light, in the neighborhood of 3600 angstroms wavelength, has proven particularly suitable; and radiations of still higherfrequency, such as X-rays and radiations from radioactive sources, arealso useful under specifically somewhat different but analogousconditions. In the latter case, shadow-graph processes are convenientlyemployed, involving irradiation of the sensitive surface through asuitable stencil held at a distance from the surface sufiicient toprovide space for the corona source and the aerosol developer. In allcases, the radiation used must be capable of producing the requireddifferential effect at the receptor surface.

Multi-colored prints are formed with ease and rapidity by the process ofthe invention, employing colored transparencies such as Kodacolornegative slides to establish the light-image, and passing the lightthrough the proper color-filters in succession while providing acorresponding succession of colored aerosols from source 14. One suchcolor-filter is indicated in FIGURE 1 by the filter it will be apparentthat such filters may equally effectively be placed between the lightsource 101 and the transparency 102. These filters are omitted wherecolor separation or intensification is not required.

It is to be understood that the term print refers to the visible orother image formed at the receptor surface by selective deposition ofaerosol developer particles, whether in the form of an immediatelyvisible deposit of ink or dye or the like, or as an invisible deposit ofmaterial which is self-reactive or visibly reactive with components ofthe receptor or with reactants from other sources and is subsequently tobe converted to visible form, or as an adherent coating for retainingsubsequently applied powders, inks, or other materials, or astransparentizing means for the receptor sheet, or in any 'oth analogousform. Pictures or photographs, made eithe u'ectly from the subject orfrom original negatives or positives, as well as reproductions ofprinted matter or drawings, are examples of prints which may be made bythe methods and with the materials and apparatus of this invention.

The corona source 11 is illustrated as a single point and is suppliedwith high voltage from a suitable source, not shown. The single pointsource is the most effective in setting up the required conditions, butmust be separated from the sensitive sheet by a distance equal to atleast approximately the diagonal of such sheet if a uniform image isdesired. Hence the use of a single point source is practical only forsmall areas. Increasing the number of such sources requires additionalcurrent and has the further disadvantage, in many instances, of formingzones in the print. The latter difficulty is experience also with someline sources of corona. Since line source or multiple-point sourcesystems permit the printing of much larger areas and at much greaterspeeds, it is desirable to substitute them for the single-point source.It has been found that such substitution is effective if the point orline sources are carefully properly spaced from each other and from thesheet, or, alternatively, if they are caused to oscillate parallel tothe surface of the receptor. Either of these procedures is found to makeavailable substantially uniform distribution of the aerosol developerparticles over the entire receptor surface. A specific example of asuitable nonoscilla-ting grid system employs a series of No. 36 B & Sgauge fine parallel wires spaced at /2 inch from each other and at oneinch from the receptor surface. The system produces a substantiallyshadow-free and zone-free print at a potential difference between thegrid and the plate of 20,000 volts.

An oscillating multiple corona source is represented in FIGURE 4. Aframe 40 is moved alternately in both lateral directions on rollers 41by means of a crank mechanism 42. The frame 40 carries a plurality ofparallel line corona source elements 43 which are connected to a sourceof potential, not shown, through connector 44. A series of spaced pointsources, or a single line source, of a coarse screen having crossed linesources and oscillating in both the vertical and the horizontaldirections, may replace the line source elements 43. The device replacesthe single point source element 11 of FIGURE 1 and is advantageouslyplaced much closer to the receptor 12 than is possible with the pointsource, thereby lowering the required voltage and reducing the spacerequirements.

The relatively high charging current requirements of the deviceillustrated in FIGURE 4 may be reduced by mechanically distributing theinput potential to each of the several corona sources independently andin sequence. Such an arrangement reduces the zoning, and eliminates theneed for accurate spacing, previously referred to.

Various volatilizable dyestuffs have been found to be well adapted tothe requirements of the process of this invention. In order to obtainthe desired high speed of printing, it is necessary that the aerosolformed of these or other materials be introduced into the space betweenthe receptor and the source of corona without delay and with maximumuniformity. Furthermore, the individual particles of the aerosol must beextremely small in order to obtain adequate definition in the print; andsmudge resistance is also improved. The present invention provides rapidcontrollable formation and uniform distribution of aerosols havingextremely fine particles by subsubstantially instantaneouslyvol-atilizing the dystuif or other active developer material from aninert carrier and with substantially no decomposition. Introducing theresulting aerosol into the desired space is readily accomplished bycontrolled convection in the apparatus illustrated in FIGURE 1.

In the apparatus of FIGURE 1, the aerosol of suspended developerparticles is produced at source 14 by electrically heating a section ofwire 141 coated with a thin layer 142 of the developer material. Thecloud of aerosol rises by convection in front of the face of the sheet12. The wire 141 with its coating 142 may be continuous, being drawnpast the indicated position and electrically activated along the portionindicated through suitable clamp means, not shown. The rate of heatingis sufficiently rapid to produce complete volatilization of the dyestulfor other developer material without any substantial decomposition.

FIGURE 5 illustrates another and preferred way in which an aerosol of adeveloper material may be produced. A belt is drawn from a supply roll51 to a windup roll 52 as needed, by mechanism not shown. The beltpasses the path of a rotatable electrically heated crank member 53. Whena charge of aerosol is desired, the belt is drawn forward the necessarydistance and the crank heated and revolved past and in brief wipingcontact with the belt. The latter is initially impregnated with adeveloper material, in this instance a volatilizable dyestuff.Immediately on contact with the preheated crank surface, the dyestuffvolatizes to form the desired aerosol, which is then drawn across theface of the receptor. The intermittent contact action of crank and beltpermits the former to reach full operating temperature before contactingthe dyestuif, so that volatilization of the latter is practicallyinstantaneous. The wiping action of crank on belt ensures a cleansurface on the crank and full contact between the .two surfaces. Theoperation may bev made entirely automatic by the incorporation ofavail-- able actuation and control means, not shown.

Moving the dye-impregnated belt over the surface of a hot wire or otherheated body is also effective.

Rapid release of large volumes of extremely fine aerosols may beaccomplished in other ways, e.g., by heating a dyestuif-impregnated beltor fabric with infrared radiation. However, the automatic systemsdescribed above have a number of advantages and are preferred.

These methods produce aerosols of unaltered dyestuffs or other developermaterials in which the individual particles are all extremely small anduniform, and in this respect differ from smokes, liquid sprays, andsimilar sources of air-dispersed fine particles. These latter materialscontain many relatively large particles, and do not provide theresolution of image nor the permanency and smudge-resistant qualitiessuch as are here made available. The extremely fine particle size of theaerosols produced may be illustrated by comparison of the settling ratesin air of these and other fine particles. Glass tubes two inches indiameter and two feet in length were charged with equivalent densitiesof air-suspended extremely fine carbon black (Darco 5-51) and of aerosolproduced by rapidly heating a resistance-wire coated with the dyestulfAlizarine Irisol N. The tubes were sealed and allowed to standvertically. Most of the carbon black settled out within about seconds,and the tube was found to be visually clear in minutes. The aerosol, onthe other hand, settled at a uniform and very much slower rate, and thetube was not visually clear until after 105 minutes of settling.Liberation of as little as 3 milligrams of dyestutf, or even somewhatsmaller amount, is suflicient to develop a 7x7 inch print in apparatusas described in connection with FIGURE 1, again illustrating theextremely fine subdivision of the dye material.

The belt source of dyestuff or other volatilizable imageproducingdeveloper material is depicted in FIGURE 3 in cross-sesction, the belt50 comprising an inert heat-resistant flexible fabric or fibrous base 31impregnated and coated with solid volatilizable dyestuff 32. Closelywoven thin glass cloth is flexible, absorbent, and heat-resistant and ispreferred as the base web.

Convection currents set up by the heating apparatus or by the heatedaerosol of volatilized dyestuif are sufficient to move the mass ofdispersed particles rapidly across the face of the receptor 12, wherethe latter is properly protected and is not unduly large. Additionalimpetus may be provided, for example by a heating wire arranged acrossthe top of the space through which the mass is to travel, where moreeffective movement is desired; or gentle air currents for transportingthe mass may be set up by other means. For example, the aerosol sourcehas been located in a duct somewhat removed from the edge of the sheet12, the aerosol then being wafted through the duct and across the faceof the sheet by means of a fan. Unused dyestuff may be recovered byfilter action or by electrostatic precipitation in order to avoidventing the material to the surroundings.

The successful formation of prints by the method and with the apparatushereinbefore described is dependent on the properties of the receptor12, which for convenience will in this description be referred to as thecopy-sheet or copyingpaper. A preferred copying-paper has aphotoelectrosensitive powder dispersed in a binder film of lowelectrical conductivity which adherently bonds the powder particles to aflexible carrier sheet having comparatively high electricalconductivity. The structure is illustrated in FIGURE 2 in which aflexible conductive carrier sheet is coated with a layer 21 of a mixtureof sensitive powder and insulating binder. The sensitive layer mayequally well be formed on relatively inflexible plates or other articleshaving a conductive surface, although for most uses the flexible carrieris preferred.

Useful papers have been produced in which the particles are incorporatedwithin the sheet, as by addition to the pulp suspension, prior to paperformation of the dis- 6 the required high proportion of particulatesensitive material, which should be present to the extent of at leastabout one-half the total weight of the dry sheet.

Sheet materials having a surface coating of the sensitive materials,while requiring additional handling, provide maximum contrast, are moregenerally applicable, and are preferred. In such cases, the particles ofsensitive ma terial are adherently afiixed to a surface of the basesheet, e.g., by means of a film-forming organic binder material havinghigh electrical resistivity. Preferred examples of suitable bindermaterials are polystyrene, chlorinated rubber, polyvinyl acetate,petroleum base diolefin resin, thermoplastic hydrocarbon resins, andpolyamide resins.

Surprisingly, somewhat less electrically resistive binders may also beused, where development of the image is accomplished, as it is in thedynamic development processes of this invention, under a continuouslymaintained potential difference. Hence the process may be carried outunder widely differing conditions of humidity etc. with no significantreduction in quality of the resulting prints.

In some cases, the sensitive material may form the entire surfacecoating, being bonded to the surface of the sheet either by anintervening binder coat or by direct contact. Thus, selenium is usefulin the form of a continuous film applied over the conductive base, forexample by vapor deposition on a metal plate or foil. Anthracene,anthraquinone, and sulfur are also effective. However, these materialsprovide undesirable color in the resulting coating, or are impermanentunder some conditions, or tend to discolor or otherwise adversely affectsurfaces of other papers or articles with which they may come in contactduring storage, or are insufiiciently sensitive to visible light, or areless desirable for other reasons.

A much preferred type of photoconductive copyingpaper, having moredesirable initial appearance as well as better storage qualities, isprepared from photoconductive materials of the class represented by zincoxide, zinc sulfide, yellow cadmium sulfide, titanium dioxide, bariumtitanate, and various phosphors such as those based on Zinc oxide, zincsulfide and cadmium sulfide, or mixtures thereof.

The backings or supporting sheet materials of this invention areelectrically conductive in comparison with the relativelyhigh-resistivity sensitive surface. Under normal humidity conditions,untreated paper is found to be usefully conductive. Regeneratedcellulose transparent film (Cellophane) is particularly effective.Somewhat increased conductance might be obtained by incorporatingconductive salts in the paper backings, for example by treating thepaper with aqueous solutions of sodium sulfate or the like; but theconductance of the resulting product varies with the humidity of thesurroundings, and background areas of prints produced on such papers arefrequently badly fogged. Non-hygroscopic particulate conductors such asacetylene black or aluminum flake have been incorporated in paperbackings to provide increased electrical conductivity; but theconductance of Such sheets is not uniform and the product may beoffcolor. Metal plates or foils are effective but are expensive,difficult to handle, and have other disadvantages.

It has been found that much superior results are obtained by providing acontinuous conductive but very thin metallic layer directly beneath acoated light-sensitive layer. Further description of the products andprocesses of this invention will accordingly be provided primarily interms of such preferred structure, which is specifically illustrated inFIGURE 2, in which the conductive backing 20 is indicated as including aflexible base 22, e.g., of paper, and a thin conductive surface coating23 which may be of vapor-deposited aluminum or very thin adherentlyattached aluminum foil. The foil-coated paper is particularly desirablesince a surface of improved smoothness is thus provided, resulting inmore accurate and sharply defined reproductions.

The metallized conductive backings as just described may be directlyelectrically connected to the Power supply, in which case the support 13need not be conductive and may be eliminated, e.g., in the case ofcontinuous strips from rolls of the sheet material, supported undertension between connecting clamps.

The adverse effect of variations in humidity on the conductivity ofcertain types of backing members is avoided in the preferred sheetmaterials of this invention as above noted. Conditions of high humiditymay have other adverse effects; for example, coatings of zinc oxide inpolystyrene binder have developed excessive background color when usedfor electrop-hotographic copying under humid conditions. It has beenfound that the adverse effects of high humidity are minimized oreliminated by the addition to the coating of small amounts ofwater-resistant metallic soaps, such as of the order of one or twopercent by Weight of zinc stearate r cobalt palmitate. The amountrequired varies directly with the humidity to which the sheet is to besubjected. These additives are found to contribute also to permanentsmoothness and lack of graininess in the photoelectrosensitive surface,and to increase the firmness of bond between the coating and theunderlying metallic surface. The polyvalent metal higher fatty acidsoaps have also been used to alter the properties of the sheet withrespect to light-sensitivity; for example, addition of small amounts ofcobalt soaps has been found to render a light-sensitive zinc oxidecoating sensitive to a much wider range of color values. Again, thesemetallic soap additives stabilize the photoelectrosensitive coatingagainst the graininess of image sometimes encountered with copy-sheetsafter prolonged aging in the absence of such additive materials.

Since the preparation of visible reproductions of printed matter such asletters, pictures, diagrams, pages of books or magazines, etc., is oneof the principal fields of utility for the present invention, thematerial used to produce the aerosol required for developing thereproduction is customarily a black or distinctly colored substance.Certain classes of dyestuffs have been found to be particularly welladapted for this purpose. They are permanently stable under normal roomand storage conditions; they do not vaporize at normal temperatures tocause discoloration of adjacent surfaces, nor do they fade or discolorunduly on exposure to sunlight. They vaporize without decomposition whenheated rapidly to operable elevated temperatures, and the resultantaerosol is readily deposited on the sensitive surface under theinfluence of the high potential. Typical dye products which have beenfound fully satisfactory for the purposes of this invention include thefollowing: Alizarine Irisol N; Brilliant Oil Blue; Anthraquinone SkyBlue B Base; Oil Red 0; Oil Yellow; Oil Brown 0; and Oil Brown N.

Oertain dyestuffs form aerosols which, when applied to some copy-sheetsas herein noted, are imperman-ently :attached and may be removed, or atleast redistributed, by rubbing or brushing. Other dyestuffs, on thecontrary, and in particular those which melt or fuse or otherwise becomeliquid, plastic, or at least somewhat sticky during volatilizat-ion, arefound to adhere strongly to the copysheet and to form substantiallycompletely smudge-proof prints. The same result has been accomplishedwith the former class of dyestuflls by adding thereto small amounts offusible material such as waxes or plasticizers. These apparently combinewith the dyestuff to produce a mixture which on volati'liz-ation is atleast temporarily tacky and therefore becomes strongly adhered to thecopysheet immediately on contact. Hence for best results in terms ofpermanent visible smudge-proof prints it is desirable to employ as thedeveloper material a dyestuff or mixture which, on volatiliz-ation,torms an at least temporarily tacky condensate which does notimmediately recrystallize or harden to a non-tacky state.

The practice of the invention is not limited to the use of vaporizabledyestuffs or colored materials, since visible images may be produced bydeposition of various chemically or physically active aerosols onproperly prepared receptor surfaces. Copying-paper prepared for suchoper ations may be more expensive and troublesome to produce, and thetime required tor development may be somewhat greater; but the processnevertheless has a number of advantages when compared to developmentwith dyestufis. The developer material may be colorless, so that it doesnot stain or discolor surfaces with which the free aerosol mayaccidentally come in contact. Delayed printing and other novel effectsare made possible.

As an example of a physical developing action, a suitable solubledyestuff is dispersed in the dorm of particles in the light-sensitivecoating and activated with an aerosol developer formed of a solventmaterial which dissolves and spreads the dyestuff to provide a visiblemark. A specific example of such a product and process employs AlizarineI-risol N as the dyestuff, uniformly and minutely dispersed in a coatingof zinc oxide particles in a polyvinyl acetate binder applied in aqueousdispersion form and then dried. The dispersed particles of the dyestuffgive only a faint tint to the coated surface. An intense visible imageis developed when the sheet is subjected to patterned deposit-ion ofres-orcin-ol in aerosol form and then warmed, the resorcinol dissolvingthe dye particles and spreading the dissolved dye to provide the visibleimage.

Re-soroinol is :a stable volatilizable solid in liquelied storm is agood solvent tor the dyestulf employed. Many other solvents, both liquidand solid, e.g., amyl or butyl monoethers of ethylene glycol,cyclohexanol, acetophenone, stearic acid, and S-hydroxyquinoline havealso been found effective. Of these, the solids are most effectivelyapplied by volatilization as herei-nbefore described, whereas usefulaerosols of the liquid materials may be produced either by rapid heatingor in other ways.

In a modification of the above-described system, the copying-paper isprovided with a strongly col-cred surface which is then mashed with asoluble protective surface coating, such as a thin microponou-s film ofblushed lacquer. The photoconductive component may be located just belowor partly within the lacquer film, but is preferably placed within thecolored layer, since in that position it is closer to the conductivebacking While still being accessible to illumination through the blushedlacquer masking layer. Activation of the sheet with a lightimjage, andapplication of an aerosol of a solvent material capable of dissolvingand rendering permanently visibly transparent and continuous thenormally visibly opaque discontinuous blushed lacquer coating, whilemaintaining a high potential on either the sheet or the electrode orboth, all as hereinbetore described, provides for the localtransparentization of the coating and exposure to view of the underlyingcolor layer in the pattern of the lightimage. With cellulose lacquers,solvent developer materials such as the volatile ethylene gyoolmonoalkyl ethers or various known non-volatile liquid or solidplasticizers are tound to be effective when applied in aerosol form.

In many instances the binder by which the photoconductive material issupported on the sheet material is also susceptible to the solventact-ion of the solvent aerosol. Dissolved :dyestuff is then caused toblend uniformly with the binder at the solvated area and imparts ahighly effective coloration to the sheet in the areas defined by thelight-image.

One important advantage of such solvent-action processes is thesubstitution, for the dye aerosol with its tendency to discolor adjacentareas, of the colorless solvent aerosol, any excess of whichsubsequently volatilizes and is harmlessly dissipated. Another advantageis the forma tion of permanent images with no possibility of subsequentsmudlg-ing.

Colored prints or copies may similarly be produced with colorless ornon-staining aerosols by a process of chemical development. AIS anexample of a chemical developing action, the light-sensitive coating isprovided with a content of ferric stearate as one reactant, andpyrogallol or catechol in aerosol form is selectively deposited thereonby methods already described, the resulting chemical react-ion betweenthe iron compound and the phenol providing a permanent visible markexhibiting a high degree of contrast. Here again, warming of the treatedsheet is frequently desirable in obtaining complete reaction of thecolor-producing components and in providing a permanent image.

Other iron soaps such as ferric laurate, myxrista-te and caprylate mayreplace the ferric ste-rate in the copyingpaper and the image may bedeveloped aerosols of iron-reactive phenols such as-8-!hydroxyquinoline, propyl gallate, protocatechuic acid, catechol,gall-ic acid, pyrogallic acid, or resoiroinol. (Typical examples ofother pairs of reactants and developers include nickel stearate anddimeflhylglyoxime; manganese, magnesium, copper, silver or cobalt soapsand phenolic materials; various heavy metal soaps with thiourea,benzotriaziol, or 2-mercapt-obenzimid-aziol; [stabilized diazonium saltsand p heiilOl'llC materials; and combinations thereof. For example, roomtemperature chemical developing action is attainable by employing anaerosol of clhlcnanil (tetra c'hlonobenzoquinone) with a photoconductivereceptor sheet having a surface coating of Dupont Hectograph White A, astabilized colorless triphenylemtihane dye.

As an alternative to the step of heating the treated sheet materialwhich has been exposed to an aerosol of a normally solid developermaterial, a high-boiling liquid solvent component may be vaporized andselectively dcposited together with the solvent or reactant material ofeither the physical or the chemical systems just described. The systemis particularly effective with copysheets containing particles ofinterreactive color-forming reactant materials. The liquid solventpenetrates the binder component of the light-sensitive coating andpermits effective contact of the chemically interreactiveimage-producing materials, but subsequently becomes dissipated withinthe much greater bulk of the binder which then seals and protects theimage.

The application of solvent-type materials in aerosol form is useful alsoin rendering substantially permanent images produced by directdeposition of dyestuffs. Volatile or normally non-volatile solvent orplasticizer materials which soften the binder component of thecopy-sheet and render it slightly sticky, or which similarly soften andtackify the developer material itself, cause the aerosol dye particlesto adhere firmly to the copy-sheet and thus provide a permanent andnon-smudging image.

Other colorless or non-staining aerosols have been used to providelithographic patterns from which large numbers of copies are printed byconventional lithographic duplication methods. For example, anorganophilic waxy developer material such as paraflin in aerosol form isapplied to pattern areas on a copying-paper having a hydrophilicsurface, such as may be obtained by rubbing the sensitized sheet with aminimum amount of finely powdered aluminum silicate. The sheet is usedin applying organic inks to paper by lithographic transfer, the oily inkbeing selectively picked up on the paraffin-coated areas and transferredto a lithographic blanket while being prevented from adhering to theadjacent water-moistened hydrophilic areas. Somewhat analogously, imagesproduced with aerosols of soluble dyes on solvent-resistantcopying-papers are transferred to paper by spirit duplication orhectographic methods. Multi-colored images are readily prepared by thislatter method.

The following examples will further serve to illustrate the principlesof the invention but are not to be considered as limitative.

EXAMPLE 1 Dense high-quality smooth-surface paper was coated on onesurface With an extremely thin layer of aluminum applied by vapordeposition under vacuum. The aluminum surface was brilliantlyreflective. Over this coating was then applied a thin continuous uniformcoating of a viscous mixture prepared by uniformly dispersing 55 partsby weight of zinc oxide powder in a solution of 5 parts of a styrenepolymer in 40 parts of toluene, and the coating was dried at moderatelyelevated temperatures. The dried layer was about one mil (.001 inch) inthickness. The styrene polymer employed was a copolymer of a predominantproportion of styrene with a small proportion of butadiene, obtainableas Pliolite S5"; it is capable of forming self-sustaining tough flexiblethin films, and is substantially nonconductive of electricity. The zincoxide, Mercks Analytical Reagent grade, had a particle size in theapproximate range of 0.1-0.5 microns or somewhat larger.

The coated sheet, about 7 x 7 inches in size, was placed in an apparatusas indicated in FIGURE 1, being supported against a grounded flat metalplate with the coated surface disposed outwardly. The image source 10was located at a distance of 36 inches from the support and consisted ofa 35 mm. Leica Prado-150 projector, operated with a watt bulb and at alens opening of about f-8. The corona source 11 Was a single pointsource located at a distance of 10 inches from the surface of thesupport. A negative potential of 50,000 volts was impressed on thepoint, the other side of the voltage source being grounded. The assemblywas maintained in dim light, and an image of a negative line drawing wasfocused on the coated sheet. An aerosol was produced at source 14 byheating the Wire 141 to substantially instantaneously vaporize thedeveloper coating 142. The developer mate rial in this case was DupontOil Brown 0, a dark brown anthraquinone dye. A short exposure,suificient only to permit the aerosol to rise past the fact of thesupport, produced an enlarged reversed reproduction of the originalnegative on the sensitive sheet, in the form of a brown line drawing ona white background.

The reproduction was characterized by extremely fine detail and sharpcontrast. The white background Was substantially free of dye, whereasthe lines of the reproduced drawing were sharply defined and uniformlydense. However, the shadow of the corona source 11 produced a visiblediscontinuity in the reproduction.

The specific developer material mentioned above produced a substantiallysmudge-proof and permanent print. Substitution of Alizarine Irisol N forthe Oil Brown 0 resulted in a print showing an undesirable degree ofsmudging when rubbed with the fingers or against a clean sheet of paper.Smudging was eliminated by blending with the Alizarine Irisol N a smallamount of paraffin as previously described herein. Smudging was equallyeffectively eliminated by adding a small amount of paraffin or parafiinoil to the binder solution used to prepare the copy-sheet. The presenceof this tackifier material at the surface of the copy-sheet served toretain the extremely fine particles of the aerosol developer, and toprevent Smudging, without producing any degree of stickiness which wasobservable to the touch or detrimental to the normal use of thecopy-sheet.

The copying-paper was held against the metal support plate by means ofspring clips having toothed edges which penetrated the surface layer andprovided for electrical contact between the metallic layer of thecopying-paper and the grounded support plate. In the absence of suchclips, effective contact is frequently formed through tiny arcs set upbetween the metal plate and the metallic layer along the edges of thesheet. Adequate contact may also be established through the thickness ofthe paper backing.

1 1 EXAMPLE 2 In this example a grid of wires replaced the point coronasource of Example 1. The grid was formed of fine wires parallel to eachother and at a distance of 2 inches from each other, the whole beingplaced at a distance of 3 inches from the front of the sensitive paper,and oscillated as shown in connection with FIGURE 4. Alizarine Irisol Nwas used as the aerosol developer material.

A photographic negative was employed as an image source, the image beingprojected and focused on the sensitive paper just prior to and duringthe evolution of the aerosol. A blue positive copy was obtained havinggood contrast and detail.

The metal support plate was then charged at 20,000 volts positive andthe grid at 20,000 volts negative from a suitable source of rectifiedA.C.

In a variation of the above, the metal plate is charged at 40,000 voltspositive while the grid is connected directly to ground. The quality ofthe print produced remains unchanged. The electrical system issimplified. The same variation is equally applicable to the systemdescribed under Example 1.

Somewhat less intense reproductions are obtained by reducing the voltageat each of the plate and grid to 10,000 volts.

EXAMPLE 3 This example duplicates Example 2, except that the source ofpotential is disconnected from the grid just before the aerosol isgenerated. The source of potential remains connected to the plate, andthe light-image remains focused on the sheet. There results a copyhaving reduced background color and more uniform distribution of theaerosol particles from the bottom to the top of the sensitive sheet,although being of somewhat diminished intensity.

EXAMPLE 4 Many photoelectrically sensitive materials have an effectivelight memory so that development may be accomplished at some periodafter the light-image has been withdrawn but while the copy-sheet isstill under the infiuence of such light-image.

The sensitive paper of Example 1 is irradiated through a photographicnegative by means of a contact printer, as used for photographicprocessing. The sheet is then placed on the vertical metal supportplate, voltage is impressed on the metal plate and on the grid, and anaerosol is generated and allowed to rise past the sensitive sheet, as inExample 2. These latter operations are conducted in semi-darkness, andseveral of the previously exposed sections may be developedsimultaneously or in immediate succession as desired. A fullysatisfactory positive print is obtained where development is carried outwithin about 10 minutes after exposure. At longer intervals theresulting copy becomes progressively weaker.

Approximately the same degree of exposure to light is used in the aboveprocesses as is found effective in typical photographic procedures. Theexposure may be obtained with a conventional contact printer or with aflashbulb or with exposures of even shorter duration, such as areobtained with stoboscopic sources of illumination used in photography.Dynamic development with the aerosol, under the influence of thecontinuously maintained high potential, may require substantiallygreater time than is required for the exposure; but several exposedsegments or frames may undergo development simultaneously, so thatexposure and development may proceed continuously and at the same rate.

EXAMPLE 5 This example describes the production of multiple color printsby the processes of this invention.

The sensitive sheet was in this case formed by coating the previouslymetallized paper backing of Example 1 with 12 a mixtureof zinc oxide andstyrene resin containing 0.01 percent, based on the weight of zinc oxideand resin, of Pontachrome Azure Blue B Conc. 200%. The dry thickness ofthe coating was about 1 /2 mils and the sheet was essentially white inappearance.

In the apparatus of Example 2, an image obtained from a blue and redtransparency through a blue-green cyan filter was developed with a blueaerosol of the dyestuff employed in Example 2, and the image obtainedfrom the same transparency through a yellow filter and superimposed onthe first image was developed with a red aerosol of Celanthrene Red YP.The two colors blended to produce an effective two-color positive printwith very little background color.

Yellow cadmium sulfide may be used in place of the combination of zincoxide and blue dye in the sensitive sheet material to produce equallyeffective multicolor sensitivity. The yellow appearance may beeffectively masked without appreciable reduction in photoconductivity byover-coating with a thin layer of titanium dioxide pigment, e.g., in apolyvinylbutyral binder.

A three-color print was similarly produced on a sheet of metallizedpaper coated with a mixture of parts of zinc oxide, 20 parts of zincoxide which had previously been treated with Azure Blue B dye, and partsof a 1:12 solution of Parlon chlorinated rubber in toluene. An alcoholsolution of the dye was added to the zinc oxide which adsorbed a portionof the dye, the remainder being removed by washing with alcohol. Otherratios of treated and untreated zinc oxide are useful, higherproportions of the treated oxide giving increased green and redsensitivity but also causing a blue tint in the sensitive coating. Thedry coating thickness was about 1 /2 mils. A colored photographicnegative of a three-color subject, such as is obtained on Kodacolorfilm, was used as the original. The image formed through a Wratten No.47-B blue filter was developed with an aerosol of Oil Yellow dye. Thesuperimposed image formed through a Wratten No. 61 (N) green filter wasnext developed with an aerosol of Celanthrene Red YP red dye. Similarly,the image formed through a Wratten No. 29 (F) red filter was thendeveloped with an aerosol of Brilliant Oil B1ue" dye. The resultingprint showed color values analogous to those of the three-color subject.

The panchromatic properties of a sheet employing a mixture of treatedand untreated zinc oxide particles in the sensitive layer are at amaximum when the treated particles carry a dye which is absorptive ofred and green light. Sheets made with zinc oxide pigment but havingincreased sensitivity to red light are produced by employing zinc oxidesensitive pigment which has been treated with a blue dye. Analogousmodifications of other sensitive particulate materials may be made.

EXAMPLE 6 In this example the apparatus and procedure is the same asdescribed under Example 2, except that, just prior to the generation ofthe aerosol, and with the light-image remaining impressed on the sheet,the voltage is reversed on both the metal support plate and the grid.Generation of the aerosol then results in a negative rather than apositive print of the negative transparency, i.e., the aerosol isdeposited on the unlighted rather than the light-struck areas. The sameprocedure may be used to provide positive prints of positivetransparencies.

Such prints may equally well be made by reversing the polarity on theplate while disconnecting or grounding the grid.

The zinc oxide of the copying-paper described in Example 1 provides awhite surface which is highly effective as a background for the coloreddyes employed in developing the copies. Other sensitizing materials maybe of equal or greater effectiveness as a lightsensitive component underexposure to a light-image, but may be much less effective as abackground component.

Cadmium sulfide, for example, is yellow-orange in color and isineffective as a background material with dyes of similar color eventhough it provides a high degree of sensitivity. It has been found,however, that a thin surface coating may be applied over a sensitivecadmium sulfide layer to give an improved print background withoutdestroying the sensitivity of the copying-paper. Thus, a minimum surfacelayer of titanium dioxide pigment in film-forming binder applied over asensitizing layer of cadmium sulfide in the same binder on a suitablebacking member results in an effective copying-paper with goodsensitivity as well as excellent background characteristics. Thetitanium dioxide is itself somewhat lightsensitive but is here employedprimarily as a background material.

Mixtures of photoconductive materials provide additional advantages overthe individual materials in many instances. One particular exampleemploys a mixture of one part of Mercks Reagent Grade zinc oxide andthree parts of New Jersey Zinc Cryptone #800 zinc sulfide as areplacement for the zinc oxide of the sheet material of Example 1. Thesheet provides greatly increased tonal range in continuous-tonereproductions produced, for example, in accordance with the proceduresof Example 2. Such a sheet may be effectively sensitized with a dye,e.g., as indicated in connection with Example 5, where panchromaticproperties are desired.

Some conductivity in the backing member is necessary in order toestablish the necessary difference in surface potential between thelocally illuminated and darkened areas of the copying-paper. The extentof current flow required is extremely minute, as may be understood fromthe fact, previously referred to, that untreated paper under normalhumidity conditions may serve as a useful base web for the copy-sheetsof this invention. Such a paper-backed copy-sheet may be clamped againsta conductive metal plate 13 as in FIGURE 1 of the drawing, but must insuch event be in close contact with the entire plate since printing isless effective in areas of poor contact. This latter difiiculty iseffectively overcome by supporting the paper copy-sheet only along theedge areas, e.g., on a frame rather than a continuous plate; which againillustrates the low degree of conductivity required in the copy-sheetbacking. Uniform printing across an entire 8 /2 X 11 inch copy-sheet isobtainable using a conductive frame covering only a one-half inchmargin. The very low current drain required for aerosol depositionpermits the sheet to be held at the charging potential over its entirearea.

As the volatilized developer material produced at the element 14 ofFIGURE 1 passes across the face of the sensitized copying-paper,particles are deposited on the sensitized surface in accordance withvariations in the light-image to which the sheet is or has been exposed.The corona discharge, e.g., from the point source 11 of FIGURE 1 or theline sources 43 of FIGURE 4, intensifies the deposition, presumably bothby charging the aerosol particles and by maintaining maximum potentialdifference at the plate surface. The added charge is desirable but notessential, as noted in connection with Example 3. A charge mayadditionally be impressed on the aerosol particles as they are firstformed, for example from a probe electrode source of corona placeddirectly above the aerosol source 14 of FIGURE 1 and below the face ofthe sheet 12, the properly charged particles then being more forcefullyattracted to appropriate areas of the exposed sheet. Such a system maybe combined with the previously described source of corona 11 or 43 toprovide a variety of results. For example, reversing the polarity of theaerosol particles by reversing the polarity of the probe electrodepermits the preparation of negative or positive prints as desired.

The light-sensitive coating which may be applied over the conductivebacking in the preferred copy-sheets of this invention comprises asensitive pigment or powder which ordinarily will be dispersed in afilm-forming binder of comparatively high resistivity. The relativeproportions of these two components should be in the neighborhood of 5:1to 20:1, these proportions being expressed in terms of weights of zincoxide and polymer as specified in Example 1. With other materials havingdifferent unit weights, the ratios are properly compared on a volumebasis. Less than approximately the mini mum indicated amount of bindermay-result in blurred backgrounds in the reproductions, although usefulcopies of line drawings have been made on copying-papers produced byapplying zinc oxide powder to a binder-coated aluminized paper sheet,with substantially no binder film being present between adjacentparticles. Excessive amounts of binder on the other hand limit thecontrast and intensity of the reproduction. Blurring is also obtainedwith coatings in which the binder component is unduly conductive, forexample in copy-sheets containing significant amounts of acidicdyestuffs or conductive salts.

The thickness of the sensitive coating is also rather critical if bestresults are to be had. With the formula of Example 1, finished coatingsof about /2 to 10-mi1 thickness have given best results. The thickercoatings are ordinarily reserved for compositions in which particles oflarger diameter are used. For example, a onemil coating is specified inExample 1 for Mercks Analytical Reagent grade zinc oxide, a Frenchprocess oxide having an average particle size in the range of about0.1-0.5 microns, Whereas oxides having a particle size up to 5 micronswould ordinarily require a dry coating thickness greater than one mil.Particles up to 25 microns in diameter have been found useful, and arebest employed in still thicker coatings. However, the energy absorptionappears to be more effective with particles in the range of 0.1-5microns, or preferably 0.14.0 microns, average diameter.

In some instances the sensitive material has been employed in theabsence of an insulating film-forming binder. Thus, zinc and cadmiumplates, or metal sheets having a surface plating of zinc or cadmium,have been converted at the surface to zinc sulfide and cadmium sulfiderespectively by appropriate treatment, and the extremely thin sulfidelayers have been found to be light-sensitive and to permit thereproduction of light-images by the methods herein described. The directapplication of the sensitive material in powder form to the paper pulpfrom which the sheet material is produced has been described previouslyherein.

The invention provides a direct method for the duplication of sketchesand drawings, typewritten correspondence, printed books and papers,photographs, and various other examples of graphic matter. It isparticularly applicable to printing of positives directly fromphotographic negatives. It provides a means of enlarging and copyingmicrofilm at rapid rates and in an effective and economical manner. Theinvention also provides a method for direct and instantaneousphotography, being applicable to the direct formation of photographs infull color as well as in monochrome and without any intermediateprocessing. Multiple photography is likewise made possible by themethods described. For example, a series of exposures on a continuoussensitized strip, e.g., a strip composed of a minimum coating of amixture of zinc oxide and zinc sulfide bonded to a transparent backingsuch as Cellophane, provides the equivalent of a cinematographic ormovie film when developed in accordance with the principles establishedin Example 4. Exposure and development require only a few seconds, andthe completed film is then immediately ready for viewing. The same istrue of photographs or copies made by X-rays or other effectiveinvisible radiation, which, for the purposes of this invention, may beconsidered as being capable of forming a light-image; the total processis extremely rapid, copies being available within seconds after theexposure and without chemical processing. Modifications of the process,particularly with respect to the type of aerosol material, make possiblethe equally rapid preparation of printing-plates for lithographic andhectographic duplicating processes; and these also may be considered asexamples of graphic representations even though the developed images maynot be directly visible. Numerous other modifications falling within theambit of the invention will be apparent in view of the disclosures andteachings here provided.

Having described our invention, we claim:

1. A flexible photoconductive copy-sheet capable of producing graphicrepresentations, comprising a flexible carrier sheet, a light-accessiblephotoconductive layer bonded to said carrier sheet, and a heavy metalsoap of a long chain fatty acid evenly distributed within at least aportion of the thickness of said copy-sheet, said metal soap beingvisibly reactive with a reactant developer material.

2. A flexible photoconductive copy-sheet capable of producing graphicrepresentations, comprising a non-metallic flexible carrier sheet, alight-accessible photoconductive layer bonded to said carrier sheet andcomprising a photoconductive zinc oxide and a film-forming binder, and aheavy metal soap of along chain fatty acid evenly distributed within atleast a portion of the thickness of said copy-sheet, said metal soapbeing visibly reactive with a reducing developer material on contacttherewith.

3. The copy-sheet of claim 2 in which said heavy metal is iron.

4. The copy-sheet of claim 2 in which said heavy metal is silver.

5. The copy-sheet of claim 2 in which said heavy metal is nickel.

6. The copy-sheet of claim 2 in which said heavy metal is copper.

7. The method for electrostatic printing which comprises projecting animage of radiation directly on to the front of a photoconductiveinsulating layer which is sensitive to said radiation and which containsa heavy metal soap carried on a support to form a differentiallyconductive pattern in said photoconductive layer corresponding to saidimage, while said differentially conductive pattern is still present insaid photoconductive layer charging a suspended reactant capable ofvisibly reacting with said heavy metal soap and directing said chargedsuspension of reactant on to the photoconductive layer in a patterncorresponding to the image for visible reaction with said firstreactant.

8. The method for'electrostatic printing which com- 5 prises projectingan image of radiation directly on to the front of a photoconductiveinsulating layer of zinc oxide and which contains a heavy metal soap asa first reactant carried on a support to form a differentiallyconductive pattern in said photoconductive layer corresponding to saidimage, while said diflerentially conductive pattern is still present insaid photoconductive layer creating an electrical corona discharge inthe atmosphere in front of the photoconductive layer at an electricalpotential difference from said support, volatilizing and charging aphenolic material as a second reactant to form a suspension of smalluniform particles, and directing said suspended particles on to thephotoconductive layer in a pattern corresponding to the image forvisible reaction with said first reactant.

9. A long-aging flexible photoelectrosensitive copysheet capable ofproducing graphic representations, comprising a nonmetallic flexiblecarrier sheet containing a continuous aluminum surface and alight-accessible photoelectrosensitive layer bonded directly to saidaluminum comprising an admixture of a film forming insulating organicbinder, Zinc oxide and in addition to said binder a small amotlnt,suflicient to impart noticeable improvement in moisture resistance, of awater insoluble polyvalent metal soap of a long chain fatty acid.

References Cited by the Examiner UNITED STATES PATENTS 1,939,232 12/33Sheppard et al. 9648 2,735,785 2/56 Greig. 3,052,540 9/62 Greig.

FOREIGN PATENTS 571,528 8/45 Great Britain.

OTHER REFERENCES 48jfoung et al.: RCA Review, December 1954, pp. 469

NORMAN G. TORCHIN, Primary Examiner.

1. A FLEXIBLE PHOTOCONDUCTIVE COPY-SHEET CAPABLE OF PRODUCING GRAPHICREPRESENTATIONS, COMPRISING A FLEXIBLE CARRIER SHEET, A LIGHT-ACCESSIBLEPHOTOCONDUCTIVE LAYER BONDED TO SAID CARRIER SHEET, AND A HEAVY METALSOAP OF A LONG CHAIN FATTY ACID EVENLY DISTRIBUTED WITHIN AT LEAST APORTION OF THE THICKNES OF SAID COPY-SHEET, SAID METAL SOAP BEINGVISIBLY REACTIVE WITH A REACTANT DEVELOPER MATERIAL.